In numerous medical procedures that involve the insertion of a medical device into a patient's tissue, e.g. minimally invasive procedures and local anesthesia, it can be of great advantage for the physician to be informed of the exact position of the medical device in the patient's tissue. For example, to introduce regional anesthesia, including peripheral nerve blocks for surgical anesthesia or postoperative analgesia, a needle can be guided to the region of interest with the help of ultrasound imaging. It has proven challenging, however, to precisely detect the needle's end point in the ultrasound image.
Northern Digital Inc., Ontario, Canada (www.ndigital.com) offers an electromagnetic detection system under the trade name “Aurora”. The system comprises a field generator for creating an electromagnetic field and various types of sensor coils that react to the field produced by the generator. One or more of the sensor coils can be embedded into a medical instrument such as a biopsy needle, a catheter or a flexible endoscope for measuring in real time the position of the instrument's tip or, if several coils are embedded, the shape of the instrument. The various types of sensor coils available differ in shape and size and can detect their position relatively to the generator's electromagnetic field in three-dimensional space and their orientation in two or three dimensions. Wires connect the sensor coils with a sensor interface unit that transmits the coils' data to a system control unit. The system control unit collects the information obtained from the sensor coils and calculates their position and orientation.
In “Evaluation of a miniature electromagnetic position tracker”, Mat. Phys. (2002), 29 (1), 2205 ff., Hummel et al. have studied the effects of the presence of an ultrasound scan head on the accuracy of the “Aurora” electromagnetic tracking system measurement results.
Placidi, G. et al. in “Review of Patents about Magnetic Localization Systems for in vivo Catheterizations”, Rec. Pat. Biomed. Eng. (2009), 2, 58 ff., distinguish between systems where the magnetic field is located outside the patient's body (“extra-body generated magnetic field” as in the “Aurora” system) and systems where the magnetic field is generated by a permanent magnet located inside the patient's body (“intra-body permanent magnet”). A system is discussed that can detect the location in three dimensions and the orientation in two dimensions of a permanent magnet that is permanently fixed to an intra-body medical device. Each measurement involves at least two spatially separated three-axis magnetic sensors in order to measure x-, y- and z-components of the magnetic field produced by the permanent magnet in at least two spatial positions. Six magnetic sensors are arranged in a circle surrounding the patient in order to ensure that each part of the patient's body is covered by at least two of the sensors. Before use, the system is calibrated to take into account the terrestrial magnetic field. In the calibration step, in the absence of the permanent magnet, the terrestrial magnetic field is measured and then subtracted from each subsequent measurement. From the remainder, the position of the magnet is calculated. It is considered a disadvantage of the system that it cannot be moved once calibrated.
Yet, the patent U.S. Pat. No. 6,263,230 B1, which is cited in Placidi et al., supra, B1 describes a “continuous automatic recalibration” scheme with which a detector can be moved after the initial calibration, even though not simultaneously with the magnet. The magnetic detector system is attached to a fluoroscopic head in a known spatial relationship to detect the position of a permanent magnet of an indwelling medical device and the magnet's field is approximated as a dipole field. In order to compensate for the terrestrial magnetic field as well as localized perturbations associated with this field, an initial calibration is performed before the magnet is introduced into the patient. For each magnetic sensor of the detector system an offset value is determined. Later, when the magnet has been introduced into the patient, the offset values are subtracted from the readings of the magnetic sensors, thus compensating for the terrestrial magnetic field and its localized perturbations. Moreover, the “continuous automatic recalibration” scheme allows compensating for the localized perturbations of the terrestrial magnetic field even if the detector system is moved: According to this scheme, the detector is moved while the magnet remains stationary at its position that is known from the previous measurement. The exact positional change of the detector is tracked by a digitizing arm and from this the magnetic field at the detector's new location due to the magnet is calculated. The result is subtracted from the field actually measured by the detector and the remainder is considered the contribution of the terrestrial magnetic field at the new location. The process can be repeated as the detector is moved to yet another location.
U.S. Pat. No. 6,216,029 B1 discloses an apparatus for ultrasound free-hand directing of a needle. Both an ultrasound probe and the needle or a needle guide are provided with orientation sensors for sensing the position of the probe and the needle with respect to a reference. The orientation sensors each may comprise three transponders in triangular alignment. The transponders preferably are electro-optical sensors which operate with infrared or visible light. Alternatively, the system comprises a magnetic transmitter and magnetic receivers attached to an ultrasound probe and the needle or needle guide. On a displays screen, the ultrasound image of a target area is shown. Moreover, the needle is shown as a distinctly coloured line, even if the needle is outside the ultrasound image. In addition or alternatively, a trajectory of the needle is displayed.
Similarly, U.S. Pat. No. 6,733,468 B1 discloses a diagnostic medical ultrasound system in which both an ultrasound probe and an invasive medical device, e.g. a cannula, have location sensors attached to them for sensing their position and/or orientation. From the positions of the needle and the probe the relative position of the needle with respect to the probe's imaging plan is determined. From this, a projected and an actual trajectory of the invasive medical device are calculated and superimposed on the ultrasound image.